initramfs: add support for multiple crypted root and swap devices

It is now possible to pass multiple crypt_roots= and crypt_swaps=
parameters (mind the "s") and have multiple devices concurring to
building the final root device and swap areas activated.
The change is backward compatible and crypt_root=, crypt_swap= is
still fully supported.

This change makes possible to support multiple device mapper block
devices grouped together into aggregated software raid devices.
For instance, two individual luks devices can be now used to build
a single raid0 or raid1 md device.

Moreover, genkernel-next initramfs now supports multiple encrypted
swap devices.